1. Field of the Invention
The present invention generally relates to wireless telecommunications, and particularly, to locating mobile units in wireless time division multiple access (TDMA) systems which utilize differential modulation or other types of phase modulation.
2. Description of the Related Art
The proliferation of mobile units and their technology and usage has revealed various applications for the ability to locate mobile units. These applications include xe2x80x9c911xe2x80x9d calls, tourist and travel information, tracking of unauthorized cell phone usage and illegal activities, and locating of commercial and government vehicles, to name a few. Conventional wireless telecommunications systems can only determine the nearest base station which typically locates a mobile unit to an accuracy of 3 to 10 miles.
However, FCC regulations, as dictated in FCC Docket 94-102, require location accuracy of about 400 feet of mobile units (cellular/PCS users) for E-911 (Emergency-911) service by Oct. 1, 2001.
Therefore, a great deal of emphasis has been placed on developing systems that can track the location of mobile units more accurately. One known method for locating mobile units is time difference of arrival (TDOA) which has been used for many years, at least in such applications as LORAN and GPS (Global Positioning System). The application of this process to cell phones includes measuring the time of arrival of the same signal (transmitted from a mobile unit) at a multiplicity of locations (base stations) and comparing the times to determine how long the signal took to reach each base station.
However, TDOA algorithm has strict requirements. For example, to use a TDOA algorithm in wireless telecommunications system utilizing antennas, at least three different base stations must be accurately synchronized in time and these base stations must simultaneously capture a known transmitted signal from a mobile unit. Only then can a TDOA algorithm can be used to compute the location of the mobile unit. Thus, one of the challenges lies in the selection of a known transmitted signal which may be simultaneously captured by a plurality of base stations. In the case of E-911 service which requires location accuracy of about 400 feet, the received signals must be time stamped with an accuracy of a few (about 400) nanoseconds to provide a sufficiently accurate computation of the mobile location. Therefore, capturing of a traditional transmitted signal which has a duration of a few microseconds (typically 41.2 microseconds) is unsuitable for E-911 purposes. The use of traditional transmitted signals will result in a location accuracy of a few miles because, as a broad principle, a wireless signal propagates at the speed of light and, consequently, there corresponds one foot of accuracy for each nanosecond in the duration. To achieve the desired location accuracy of 400 feet, a moment within a transmitted signal (an event) or a phase change (an event) within a signal must be captured. This event must be very specific in nature and must be agreed upon by all of the base stations involved in the location determination.
An apparatus and method for generating and measuring a known event having a duration of a few nanoseconds are provided.
The present invention provides an improved phase demodulation technique for use with quadrature phase-shift-keyed (QPSK) signals and other types of phase-modulated signals in a communication system. This phase demodulation technique generates a xe2x80x9csignature eventxe2x80x9d which is based at least in part on frequency information generated by frequency demodulation of the phase-modulated signal. This signature event has the duration of a few nanoseconds and has resistance to gain and noise variations.
In an illustrative embodiment, a phase-modulated signal is separated into first and second portions. The first portion is then phase demodulated to generate demodulated symbols, while the second portion is frequency demodulated to generate a measure of the instantaneous frequency of the phase-modulated signal. The instantaneous frequency measure is then processed to identify one or more symbol transitions. The identified transitions are then used to generate an event which is so precise and unique that it has xe2x80x9csignaturexe2x80x9d properties.
This signature event can be used by base stations as an agreed-upon event to be captured and used in TDOA algorithms for mobile unit location determination. The use of this signature event results in a location accuracy of a few meters.